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Mecanorreceptores

Mechanoreceptors are sensory receptors responsible for relaying extracellular mechanical stimulus to intracellular signal transduction through mechanically-gated ion channels. Mechanical stimuli can be touch, pressure, sound waves, stretching, or motion. There are four classes of mechanoreceptors: tactile receptors, proprioceptors, hair cells, and baroreceptors.

Puntos clave sobre los mecanorreceptores
Definition Mechanoreceptors are specialized sensory cells that relay mechanical stimuli into electrical signals and transmit them to the central nervous system (CNS).
Adaptation The ability of mechanoreceptors to adjust their response to a sustained stimulus over time.
Tactile receptors Epithelial tactile complex (Merkel cell-neurite complex): Slowly adapting mechanoreceptors located in the basal layer of the epidermis responsible for prolonged light touch.
Tactile (Meissner) corpuscles: Rapidly adapting mechanoreceptors of glabrous skin responsible for precise manipulations with fingertips.
Bulbous corpuscles (Ruffini endings/corpuscles): Slowly adapting mechanoreceptors responsible for skin stretching, movement, and finger position.
Lamellar (Pacinian) corpuscles: Rapidly adapting mechanoreceptors involved in discriminating fine surface textures and sense vibration.
Hair follicle endings: Sensory nerve endings associated with hair follicles and involved in tactile sensation and thermoregulation.
Proprioceptors Mechanoreceptors responsible for the position, movement, and spatial orientation of the human body. This group of mechanoreceptors includes muscle spindles, Golgi tendon organs and joint receptors.
Hair cells in internal ear Specialized sensory cells responsible for the detection of mechanical stimuli arising from sound vibrations and changes in head position and movement.
Baroreceptors Mechanoreceptors relaying information about blood pressure within the autonomic nervous system (ANS).
Contents
  1. Adaptation
    1. Rapidly adapting or phasic receptors
    2. Slowly adapting or tonic receptors
  2. Tactile receptors (Cutaneous mechanoreceptors)
    1. Epithelial tactile complex (Merkel cell-neurite complex)
    2. Tactile (Meissner) corpuscles
    3. Bulbous corpuscles (Ruffini endings)
    4. Lamellar (Pacinian) corpuscles
    5. Hair follicle endings
  3. Proprioceptors
  4. Internal ear hair cells
    1. Cochlear hair cells
    2. Vestibular hair cells
  5. Baroreceptors
  6. Low and high threshold mechanoreceptors
  7. Sources
+ Show all

Adaptation

Adaptation of mechanoreceptors refers to the ability of these receptors to adjust their response to a sustained stimulus over time. Adaptation in mechanoreceptors involves both mechanical and electrochemical mechanisms. 

Mechanical components, like the capsule, act as filters, selectively allowing dynamic components of mechanical stimuli to reach the nerve endings while reducing static components. When the capsule is removed, as proven in dissection experiments, the nerve ending's response to sustained stimuli is notably prolonged. 

Moreover, electrochemical processes related to action potential encoding play a major role in producing adaptation. Depending on the duration, there are two main types of adaptation: rapid and slow.

Rapidly adapting or phasic receptors

Rapid adaptation refers to the rapid decrease of mechanoreceptors' response to a sustained stimulus. This allows them to ignore unchanging cutaneous sensory input and mainly detect changes in stimuli.

Slowly adapting or tonic receptors

Slow adaptation refers to a gradual decrease in response over time. Slowly adapting mechanoreceptors continue to respond to a sustained stimulus but with reduced sensitivity, allowing them to maintain some level of responsiveness to ongoing stimuli.

Tactile receptors (Cutaneous mechanoreceptors)

Tactile receptors are present in both the superficial and deeper layers of skin until near the bone. These receptors are low-threshold mechanoreceptors innervated by relatively large myelinated axons (type Aβ), ensuring the rapid central transmission of tactile information.

There are five major categories of tactile mechanoreceptors: epithelial tactile complexes (Merkel cell-neurite complexes), tactile corpuscles (Meissner corpuscles), bulbous corpuscles (Ruffini endings), lamellar corpuscles (Pacinian corpuscles) and hair follicle receptors.

Epithelial tactile complex (Merkel cell-neurite complex)

Tactile epithelial cells, commonly known as Merkel cells, are associated with sensory neurons. These mechanoreceptors are located in the basal layer of the epidermis

They are slowly adapting mechanoreceptors responsible for prolonged light touch and are mainly found in areas with high tactile sensitivity, such as the lips, palms, soles, oral cavity and hair follicles. The combination of Merkel cells and their associated afferent nerve terminals are referred to as an epithelial tactile complex (Merkel cell-neurite complex).

Tactile (Meissner) corpuscles

Tactile corpuscles, otherwise known as Meissner corpuscles, are rapidly adapting mechanoreceptors located in the dermal papillae of glabrous skin.

They are encapsulated nerve endings consisting of elongated Schwann cells, a connective tissue capsule and a central axon. They are highly sensitive and relay fine touch and low-frequency vibration sensations between 10 and 50 Hz. Because of their high sensitivity, tactile corpuscles are responsible for precise manipulation with fingertips, such as reading Braille.

Bulbous corpuscles (Ruffini endings)

Bulbous corpuscles or Ruffini endings are slowly adapting, encapsulated mechanoreceptors located deep in the skin, ligaments and tendons

They are elongated, spindle-shaped specializations with their long axis usually oriented parallel to the stretch lines in the skin. Thus, they are primarily responsible for skin stretching, movement and finger position. They account for about 20% of the receptors in the human hand.

Lamellar (Pacinian) corpuscles

Lamellar corpuscles, or else Pacinian corpuscles, are rapidly adapting mechanoreceptors located in the subcutaneous tissue and also in interosseous membranes and mesenteries of the gut. 

They have an onion-like capsule that surrounds one or more afferent axons. The capsule acts as a filter, allowing only transient disturbances at high frequencies (250–350 Hz) to activate the nerve endings. The low response threshold and the rapid adaptation suggest that Pacinian corpuscles are involved in discriminating between fine surface textures and sensing vibration.

Hair follicle endings

Sensory nerve fibers densely innervate various types of hairs in mammals. The arrangement and distribution of these nerve endings are important in tactile sensation and thermoregulation.

Proprioceptors

Proprioceptors are mechanoreceptors responsible for continuously providing information about the position, movement, and spatial orientation of our muscles, tendons, and ligaments within a three-dimensional space. This group of mechanoreceptors includes muscle spindles, Golgi tendon organs, and joint receptors.

  • Muscle spindles are located in the belly of skeletal muscles and sense the stretching or contraction of the muscle fibers.
  • Golgi tendon organs are located at the myotendinous junctions, situated between the extrafusal skeletal muscle fibers and their associated tendons. They sense changes in muscle tension within the tendon caused by muscle contraction.
  • Joint receptors sense limb position and joint movement. The function of these joint receptors is not yet well understood.

Internal ear hair cells

These mechanoreceptors are specialized sensory cells responsible for detecting mechanical stimuli, such as sound vibrations and changes in head position and movement, playing critical roles in both hearing and balance. The internal ear contains two main types of mechanoreceptors: cochlear hair cells and vestibular hair cells.

Cochlear hair cells

Hair cells are the primary mechanoreceptors responsible for detecting sound waves and converting them into electrical signals that the brain can interpret as sound. Cochlear hair cells are located within the spiral organ (organ of Corti). When sound vibrations enter the cochlea, they cause the hair cells' stereocilia, which are tiny hair-like projections, to bend, triggering the release of neurotransmitters and generating neural impulses that are transmitted to the brain via the vestibulocochlear nerve.

Vestibular hair cells

In addition to hearing, the internal ear is also responsible for sensing balance and spatial orientation through a set of vestibular receptors. These receptors detect movements and changes in head position, including linear acceleration and rotational motion. Vestibular receptors include the utricle of vestibular labyrinth, saccule of vestibular labyrinth, and three semicircular canals. Within these structures, specialized vestibular hair cells detect changes in the position and movement of tiny calcium carbonate crystals called otoliths, as well as the movement of fluid within the semicircular canals. This information is then transmitted to the brain to maintain balance and stabilize vision during head movements.

Baroreceptors

Baroreceptors are mechanoreceptors relaying information about blood pressure within the autonomic nervous system (ANS). They are mainly located at the aortic and carotid sinuses. When blood pressure is low, baroreceptors are inactive. But, when blood pressure increases, the aortic and carotid sinuses stretch, leading to the activation of baroreceptors. The frequency of baroreceptor action potentials is equivalent to the measure of blood pressure. The action potentials travel to the nucleus of solitary tract within the medulla oblongata

Increased activation of the nucleus of solitary tract leads to inhibition of the vasomotor center and stimulation of the vagal nuclei. This process inhibits the sympathetic system and activates the parasympathetic nervous system.

On the other hand, low blood pressure leads to a reduction in stimulation of the nucleus of solitary tract, causing an increase in sympathetic stimulation. This is the baroreceptor reflex which causes increased cardiac output and vasoconstriction.

Low and high threshold mechanoreceptors

Depending on the range of mechanical stimuli detection, tactile (cutaneous) mechanoreceptors are divided into two groups:

  • Low-threshold mechanoreceptors react to light, benign pressure. All the above-described tactile receptors fall into this category and are associated with large myelinated Aβ nerves. They are further classified into two groups based on their adaptation to sustained stimulus: phasic and tonic receptors.
  • High-threshold mechanoreceptors are part of the nociceptors and react to stronger, harmful mechanical pressure. They are free nerve endings that extensively innervate the epidermis. Free nerve endings are unspecialized and can function as nocireceptors (respond to pain), mechanoreceptors (respond to displacement), or thermoreceptors (respond to temperature). Nociceptors, for example, have thin myelinated (Aδ) and unmyelinated (C) nerve fibers that mediate mechanical pain.

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